Apparatus for correcting misprinted matter on sheet material

ABSTRACT

Apparatus for correcting misprinted matter on sheet material, the means for directing a stream of finely divided particulate abrasive or granular material on the sheet material and means for collecting the spent material immediately after use, in which the projection may take place within an enclosure which is positioned over the sheet material and to which a suction air flow is connected to remove the abrasive material after use.

United States Patent 1191 1111 3,715,838

Young et a1. Feb. 13, 1973 APPARATUS FOR CORRECTING [56] ReferencesCited MISPRINTED MATTER ON SHEET UNITED STATES PATENTS 2,448,316 8/1948Lesavoy ..5l/14 X [75] Inventors: Gideon Maurice Young, London;2,597,434 5/1952 Bishop et aL. ..5l/l2 Harold Silman, Esher; Eric John2,810,991 10/1957 Mead et 81.... "51/8 n Slough all of England 3,545,99612/1970 Duncan ..5 1/3 10 X [73] Assignee: Vacu-Blast Limited, Slough,En- R I N PATENTS OR APPLICATIONS gland 839,265 6/1960 Great Britain .519 22 Filed: March 2, 1971' Primary Examiner-Donald G. Kelly [21] Appl'120248 Atl0rneySteward & Steward [30] Foreign Application Priority Data1 ABSTRACT March 6,1970 Great Britain ..10,837/70 Apparatus forCorrecting misprinted matter On Sheet March 26, 1970 Great Britain..l4,684/70 material the means directing a Stream of finely videdparticulate abrasive or granular material on the 52 U.S. c1 ..s1/12Sheet material and means for collecting the Spent [51] Int Cl 1324c 1/00material immediately after use, in which the projec- [58] Fieid llllll319 320 tion may take place within an enclosure which is posillllll utioned over the sheet material and to which a suction air flow isconnected to remove the abrasive material after use.

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6b 2 b2 I 84 63 8 64 FIGS l INVENTOR ERIC JUHN HILL ETAL 87M ATTORNEYSAPPARATUS FOR CORRECTING MISPRINTED MATTER ON SHEET MATERIAL Thisinvention relates to apparatus of the kind in which granular or abrasivematerial is directed from a storage hopper on to a surface to betreated. In particular, though not exclusively, it relates to apparatusfor correcting misprinted matter or set-off on sheet material.

In the practice of printing there are frequent occasions when errorsoccur on printed sheet material such as documents and the like, whichgenerally means that the matter is wasted and becomes useless. In somecases an erratum slip is attached to the printed article or over-printedthereon, but this is by no means a satisfactory solution to the problem.Occasionally, when an error occurs on a limited number of printeddocuments or books, it is possible to erase the misprint by hand by theuse of a rubber, abrasive powder, or by coating the error with a whitematerial, and then overprint, but this is a slow, costly, and tediousprocedure. Another problem occurs with set-off which causes a smudge ofink to be picked up on the paper during printing, and which must beremoved, especially on printed items of high quality. The object of theinvention is to satisfy a need for a method of removing print from paperat high speed automatically or regulated in some manner withoutsubstantially damaging its surface, in order that it may then beover-printed with the correct words, diagrams, illustrations or othermatter.

Theinvention consists in a method for correcting a misprint. Theinvention pertains to an apparatus for correcting misprinted matter onsheet material comprising means for directing a stream of finely dividedparticulate abrasive material on the sheet material and means forcollectingthe spent material immediately after use. The invention stillfurther consists in an apparatus as set forth in the preceding paragraphin which the stream of particulate material is projected within anenclosure which is positioned over the sheet material and to which asuction air flow is connected to remove the abrasive material after use.The invention still further consists in an apparatus as set forth in thepreceding paragraph in which the particulate material is projected at apre-determined velocity and angle to the surface of the sheet materialin a stream of compressed air through an accelerating nozzle.

The invention still further consists in an apparatus as set forth in thepreceding paragraph in which the means for directing abrasive materialentrained within a stream of air comprises a storage hopper for theabrasive material, which is closed below by a porous plate and above bya filler closure, a chamber below the porous plate with means forconnection to a source of air under pressure, pressure air operatedvalve means mounted across the hopper just above the porous plate andcomprising of first cylinder connected at one end to a second cylinderhaving a piston with a piston rod co-axial of the first cylinder, aco-axial bore in the cylinder opposite to the second cylinder, which isengageable by the end of the piston rod to close the latter, a radialbore in the first cylinder for the passage of abrasive material from thehopper to the first cylinder, a channel for the passage of air from thefirst cylinder to the hopper above the level of the abrasive materialtherein, duct means associated with valve means connected on oppositesides of the piston in the second cylinder for opening and closing thevalve, and a nozzle connected by channel means to the co-axial bore inthe first cylinder.

The invention still further consists in an apparatus as set forth abovein which the means for directing abrasive material entrained within astream of air comprises a storage hopper for the granular material whichis closed below by a porous plate and above by a filer enclosure, achamber below the porous plate with means for connection to a source ofair under pressure, a channel means connected to a source of pressurepassing through the storage hopper above the porous plate with arestrictor orifice co-axially inside the channel, a further orificethrough the wall of the channel for the passage of abrasive materialfrom the hopper into the channel, and a nozzle connected to the outletend of the channel means.

The invention still further consists in apparatus as set forth in thepreceding paragraph in which the nozzle is mounted in an enclosure ofbox like form and has a bottom with an aperture therein for the passageof the stream of granular material therefrom on to sheet materialpositioned adjacent to and below the bottom of the enclosure.

The invention still further consists in apparatus as set forth in thepreceding paragraph in which an air pervious seal ring is providedaround and below the aperture in the bottom of the enclosure.

The invention still further consists in apparatus as set forth in thepreceding paragraph in which the enclosure is provided with a slot inopposite sides thereof, the one open to atmosphere while the other isadapted to be connected to the fan of a dust collector unit.

The invention still further consists in apparatus as set forth abovemounted so as to be readily moveable relative to the sheet material soas to direct the stream of abrasive material on to the desired pointupon the sheet material.

The accompanying drawings show, by way of example only, a number of theembodiments of the invention in which:

FIGS. 1, 2 and 3 show diagrammatically in cross-section threeembodiments of the invention;

FIG. 4 shows a plan view of a complete apparatus;

FIG. 5 shows a side elevation of the apparatus of FIG. 4;

FIG. 6 shows a rear elevation of the apparatus of FIG. 4;

FIG. 7 shows a sectional elevation in detail through the storage hopperof blast enclosure of FIG. 4; while FIG. 8 shows a sectional elevationin detail through the storage hopper and blast enclosure in a furtherembodiment.

The particulate material is directed on to the paper A, at a finiteangle to it, either by conveying it in a compressed air stream B,through an accelerating nozzle C, or centrifugally directing it by meansof a multi-bladed wheel D, into which the material is fed. Immediatelyafter impingement on paper, the spent granular material is removed bysuction. This may be achieved by means of a recovery annulus E, closelysurrounding the accelerating nozzle as shown in FIG. 1, a separaterecovery nozzle F, close to and at an angle to the accelerating nozzleor multi-bladed wheel, as shown in FIG. 2, or finally, by mounting theaccelerating nozzle or wheel inside a viewing cabinet G, through which arecovery air flow H, is induced, as shown in FIG. 3.

Where the recovery annulus is used, surrounding the acceleratingnozzle,-an air pervious ring I, may seal the annulus against the paperbeing treated. This ring may consist of a bristle brush, or porousplastic, etc., to prevent the particulate material from escaping intothe surrounding atmosphere but at the same time allow air to be drawninto the annulus.

Amongst the abrasives which can be employed are angular aluminous oxide,silicon carbide, etc., although the invention is by no means limited tothe use of these substances. The most suitable range of size for theabrasive is between 6-16 microns and 19-47 microns, i.e. sizes 600 and320. It is projected on to the'paper at an angle of from 90 to to itsplane by means of compressed air preferably at a pressure of 10 to 50lbs per square inch or, alternatively, centrifugally propelled. It hasbeen found that the more acute the angle is to the plane of the paper,the more rapidly will the print be removed, although there is a dangerthat the surface of certain types of paper may be more liable to damageif the angle at which the abrasive is directed is excessively acute. Ithas been found that by using 400 mesh alumina powder directed at anangle of 4050 to the surface, it is possible to remove black print fromhigh quality coated paper in 2 to 3 seconds using compressed air at 30lbs per square inch pressure. Where large areas of print are to beremoved, a means is provided to enable the means for propelling theabrasive and the suction nozzle to be traversed across the paper.

In its normal form, the machine for carrying the process out in thisinvention consists of a means of transferring sheets of paper or thelike singly on to a flat bed as is the common practice in printing,after which the abrasive is projected on to it from a stationary ormoving source. In the latter case, the speed of movement of the abrasivestream across the paper source can be up to 100 feet per minute,although a speed of 10 to 30 feet per minute is usually adequate.

Alternatively, it may only be required to delete a small spot forcorrection. This can be most readily achieved by mounting the blast andrecovery means on an arm which can be raised and lowered on to the sheetfor spot blasting. The sheets can either be stocked in a pile and thetop sheet removed between each spot blast or, alternatively, the sheetscan be fed into position against a register between each spot blast.

In such cases, when an accelerating nozzle is used, the nozzle bore canbe as small as .0.25mm to provide a narrow blast pattern. Where a largeblast stream is produced such as with centrifugal acceleration of theparticulate material, a stencil .1 made of rubber, steel or othersuitable material can be applied to the paper to restrict the area on towhich the particulate matter is projected.

After the misprinted matter has been removed, the paper may beover-printed correctly. This can be carried out on a separate machine,but on continuous applications the print erasure and over-printedoperations can be carried out more conveniently on the same machinesuccessively, by synchronizing the paper feeding, particulate materialapplication and over-printing processes, so that a sufficient time isallowed for the misprinted matter to be removed.

A further embodiment consists of a storage hopper 1, see FIGS. 4-7,within which is stored abrasive material 2. Beneath the storage hopperis mounted a feed valve 3, which meters a controlled quantity of theabrasive material into an air stream in which it is conveyed along ablast hose 4, made of rubber, plastic, or similar flexible material andthrough a nozzle 5. This accelerates the flow of air and abrasivematerial and directs it in a concentrated stream 6, towards a sheetmaterial 7, being treated. In order to obtain a very small stream forthe rectification of single spots, the nozzle may be as small as 025mmin bore.

The nozzleS, is positioned inside an enclosure 8, to retain the streamof air and abrasive and prevent it spreading beyond the particular areabeing treated. Connected to the enclosure is an outlet duct 9, whichmay, for instance, being made of a circular or rectangular tube, with atransition piece 10, forming a recovery slot 11, across the width of theenclosure. This outlet duct 9, is connected to a suction air flow toremove abrasive material and debris from the enclosure. One convenientmethod is to connect a Dust Collector/Fan unit (not shown) to the end ofthe outlet duct 9, by means of a flexible vacuum hose 12, FIG. 5. In theopposite face of the enclosure to the recovery slot is an air entry slot13, which provides a flow of air at high velocity across the base of theenclosure to remove the spent abrasive material immediately after impacton the sheet material.

Referring now to the storage hopper 1, this is fitted at the top with aquick-acting filler plug 14, for charging with abrasive material.Beneath the hopper is a cap 15, which retains a porous disc of ceramicor sintered bronze etc., 16, which forms an inner base of the hopper.The porosity of the disc must be such that, while permitting air to flowthrough, it will not allow the passage of the abrasive material.

Passing horizontally through the storage hopper at a point just abovethe porous disc is a feed tube 17, containing a stepped bore 18 (FIG.7). Into the smaller (outlet) bore is fitted the blast hose 4, which islocked by means of a compression ring 19, so that the hose end justprojects into the larger (inlet) bore. The inlet end of the feed tube17, is connected to an adaptor elbow 20, into which in turn is connectedan air cylinder 21. A piston rod 22, of a piston in the air cylinder hasa conical or similarly shaped projecting end, and is of such a lengththat when fully extended it seals against the end of the blast hose 4.

In operation, compressed air is admitted through hose 23, via the porousdisc 16, into the storage hopper 1, which is thus pressurized. Withcompressed air supplied to the air cylinder rear hose 24, the piston rod22, is extended to seal against the end of the hose 4, so that aircannot flow from the storage hopper. When hose 24, is exhausted, and theair cylinder front hose 25 is pressurized, the piston rod retracts andair then flows from the storage hopper through hose 26, along the feedtube 17, and hose 4, and finally through the nozzle 5. This results in avertical flow of air through the storage hopper which fluidizes the veryfine abrasive material, which may be as small as l0 microns in size, andenables it to flow freely. The degree of fluidization achieved iscontrolled by the vertical air flow which is determined by the bore ofthe nozzle 5. The porosity of the disc 16, and dimensions of the hopper1, can also be the feed tube 17, allows the abrasive material to flowinto the feed tube and to be carried in the air flow and acceleratedthrough the nozzle 5.

The size of the orifice 27, is selected to provide the correct flow ofabrasive material, and on extending the piston rod 22, to contact thehose 4, instant cessation of air and abrasive material flow results.Conversely, by mounting the storage hopper adjacent to the enclosure thelength of the hose 4, is kept to a minimum. Thus, when the piston rod isretracted, the flow of air and abrasive material through the nozzle isvirtually instantaneous and without surging due to excessive abrasivelying in the hose 4. It is important that, when retracted, the pistonrod is well clear of the orifice 27, so that abrasion of the piston rodby the flow of abrasive material is eliminated.

The hose 4, and nozzle 5, can be conveniently mounted in the enclosureinside a tubular holder 28, and set to direct abrasive at a convenientangle on to the sheet material through an aperture 29, in the baseplate30, of the enclosure. For many applications in the removal of misprintedmatter it is desired to restrict the abrading action to a very smallarea. For this purpose the holder 28, is fixed to direct abrasivematerial through a very small orifice 31, in a stencil 32, made ofabrasion resistant rubber or plastic. The stencil can be readilyreplaceable to suit the area being treated and retained in position by asimple clip 33.

If it is required to erase more than a single spot from the sheetmaterial, the holder 28, can be flexibly mounted, for instance in arubber diaphragm 34. This enables the nozzle 5, to be directed over alarge area around its mean position by operation of the handle 35. Forall methods of operation in order to enable the erasing action to bereadily accessed a viewing window 36, is provided in the top of theenclosure.

In operation, the enclosure baseplate 30, may be laid directly on to thesheet material 7, when abrasive material is directed through the nozzle.With such an arrangement fine dust from the erasure operation maypenetrate locally between the baseplate and the sheet material aroundthe aperture 29, although this is not normally detrimental to subsequentcorrection. However, by attaching a seal 37, of air pervious materialsuch as felt around the aperture, air is drawn locally into theenclosure to scavenge any dust, although the abrasive material will notbe able to escape from the enclosure. Theair entry slot 13, can then berestricted in area or possibly eliminated completely so that all airenters via the aperture 29.

An alternative embodiment of the invention shown in FIG. 8, consists ofa storage hopper 58, within which is stored abrasive material 59. Withinthe storage hopper is a feed valve 60', which meters a controlledquantity of the abrasive material into an air stream in which it isconveyed along a blast hose 61, made of rubber, plastic or similarflexible material and through a nozzle 62. This accelerates the flow ofair and abrasive material and directs it in a concentrated stream 63,towards a sheet material 64, being treated.

The nozzle 62, is positioned inside an enclosure 83, to contain thestream of air and abrasive and prevent it spreading beyond the areabeing treated. Connected to the enclosure is an outlet duct 65, whichmay for instance be made of a circular or rectangular tube with atransition piece 66, forming a recovery slot 67, across the width of theenclosure. This outlet duct 65, is connected to a suction air flow toremove abrasive material and debris from the enclosure. One convenientmethod is to connect a dust collector/fan unit (not shown) to the end ofthe outlet duct 65, by means of a flexible vacuum hose as 12, in FIG. 5.In the top face of the enclosure is an aperture 68, which permits theflow of air to enter at a velocity sufficient to prevent upward escapeof abrasive material and thus remove the spent abrasive materialimmediately after impact on the sheet material. The aperture 68, alsoserves as a viewing aperture so that the effect of the abrasive impactcan be observed.

Referring now to the storage hopper 58, this is fitted at the top with afilling cap 69, for charging with abrasive material. In the base of thehopper a porous disc of ceramic or sintered bronze, etc., 70, isretained by means of bolts or similar devices against a rubber orsimilar seal 71. The porosity of the disc is such that while permittingair to flow through, it will not allow the passage of abrasive material.Passing horizontally through the storage hopper at a point just abovethe porous disc is an inlet tube 72, and the blast hose '61. These arelocked in position by means of nuts 73, and flexible rings 74, toprevent the escape of abrasive material. Between the tube 72, and hose61, is fitted the feed valve 60, containing an orifice 75, to controlthe flow of air along the tube 72. A secondary tube 76, with an orifice77, causes abrasive material to flow from the hopper 58, at a controlledrate into the tube 76, and along the hose 61, to the nozzle 62, where itis accelerated and directed on to the sheet material.

While in operation, compressed air is admitted through the pipe 78, at acontrolled rate such that it passes through the porous disc 70, andfiuidizes the abrasive material 59. The abrasive material will then flowreadily through the orifice 77, whenever compressed air is admitted tothe tube 72, and flows through the feed valve 60, and hose 61. In orderto prevent the air flow through the porous disc from pressurizing thestorage hopper a hose 87, is connected from the hopper at a point abovethe level of the abrasive material and into the outlet duct 65.

The orifice 77, can be positioned anywhere around the feed valve 60, andis of -a size selected to provide the correct flow of abrasive material.

As indicated in the arrangement in FIG. 8, the hose 61, can be rigidlyattached to the enclosure 88, and set to direct the stream of abrasivematerial at a convenient angle on to the sheet material through anaperture 79, in the base plate 80, of the enclosure. As earlierdescribed, a stencil 81, may be employed, retained in position with aclip 82, and with an orifice 83, to limit the effect of the blast streamon the paper to a specific area.

The nozzle can of course be hand manipulated to be directed over theentire area of the aperture 79, being readily viewable through the upperaperture 68. Again as earlier described, an air pervious seal 84, may beaffixed to the base of the enclosure around the aperture to preventescape of dust or abrasive material on to the surface of the sheetmaterial. It has been found that by shaping the front edge of the rearseal as shown by 85, and omitting the seal across the front edge of theaperture, some of the air drawn through the slot 67, is able to enterthe base slot $6, and more effectively sweep dust and abrasive materialfrom the sheet material.

The construction shown in FIG. 8 has applications relating to abrasiveblasting generally for example for the industrial cleaning of componentparts, with or without an enclosing cabinet.

In order to be able to direct the abrasive material on to any particulararea of the sheet material 7, or 64, it is essential that the enclosurebe able to be moved readily over the sheet material. it must also becapable of being locked in a pre-determined position, when correcting abatch of sheets. Thus as each sheet is transferred into position fortreatment, the same area will be erased for correction. At the sametime, it is required for the enclosure to be raised from the sheetmaterial for the treated sheet to be removed and a new sheet inserted.Finally, for simplicity of operation, the control of the stream ofabrasive material should be automatically operated by the lowering ofthe enclosure on to the sheet material. These requirements can beachieved in a variety of manners, and one suitable system is thatindicated, which operates as follows:

Positioned around the outlet duct 9, is a lower guide tube 38, (FIGS.4-6) which may be of nylon or similar low friction material, and rollers4%. These are so adjusted that the guide tubes moves freely over theoutlet duct but with the minimum of side movement. The enclosure 8, canthus be extended to reach fully over the sheet material, and be lockedin position by the screw 41, which grips the outlet duct in the lowerguide tube.

Rigidly attached to the top of the lower guide tube, but at 90 to it, isan upper guide tube 42, which is similarly fitted with side guides 43,and rollers 44. This moves over a transverse bar or tube 45, ofsufficient length that the enclosure 8, can fully traverse the width ofthe sheet material. The upper guide tube can then be locked in positionby the screw do, which grips the transverse tube.

The ends of the transverse tube are adapted to locate in two flangemounted ball bearings 47, attached to the vertical end plates 4%, of thebase frame 49. This enables the transverse tube to rotate about itslongitudinal axis which causes the enclosure to be raised or loweredfrom the sheet material. Connected to one end of the transverse tube isa crank arm 50, attached by means of a pin 51, to a fork 52. The fork isfitted to the piston rod or an air cylinder 53, which in turn is securedon a trunnion 54. Thus as compressed air is admitted to a lowerconnection 55$, the piston rod extends, rotates the transverse tube 45,and raises the enclosure from the sheet material. Conversely, ascompressed air is admitted to upper connection 56, the enclosure islowered into hard contact with the sheet material.

It is a simple matter to co-ordinate the movement of the enclosure andflow of abrasive material by a pneumatic or electro-pneumatic controlsystem to give the following sequence of operation with reference toFIGS. 5-7. When the control system is switched on, compressed air isadmitted to hoses 23, 24, and 55. This pressurizes the storage hopper i,but closes the feed valve assembly 3, to prevent any flow of air andabrasive material through the nozzle 5. At the same time, the enclosureis raised clear of the sheet material. By slackening the screws 41, and46, the enclosure can bemoved in two directions so that when lowered,the stream of air and abrasive material erases any selected region onthe sheet material 7. A foot operated control valve can conveniently beemployed which exhausts hose 55, and pressurizes hose 56, and after apreset delay period exhausts hose 24, and pressurizes hose 25. Thiscauses the enclosure to be lowered on to the sheet material after whicha flow of air and abrasive material takes place through nozzle 5.Conversely, when the foot operated control valve is de-actuated, thefeed valve air cylinder 21, first closes to stop the blast stream, andafter a preset period the air cylinder 53, raises the enclosure from thesheet material 7. It is thus only necessary to arrange an automatic feedof sheet material between the actuation of the foot operated valve, toremove a misprinted area from a batch of sheets. Due to the suctionapplied to the enclosure to recover the abrasive-material there may be atendency for the sheet material to adhere to the enclosure as it israised. This can be overcome by a simple spring plunger 57, located eachside of the enclosure and extending freely a short distance below thebase plate 30.

Although the above describes one system for the application of ourinvention, many variations are possible without departing from the basicprinciple involved.

It is to be understood that the above description is by way of exampleonly and that details for carrying the invention into effect may bevaried without departing from the scope of the invention claimed.

We claim:

1. An apparatus for correcting misprinted matter on sheet material,comprising means for directing a stream of finely divided particulateabrasive material entrained within a stream of air onto the sheetmaterial, and means for collecting spent material resulting fromimpingement of said abrasive material against said sheet material, anenclosure positionable over said sheet material within which said streamof particulate abrasive material is projected, and means including asource of suction air flow connected to said enclosure; said means fordirecting said abrasive material comprising a storage hopper for saidabrasive material, closure means for said hopper comprising a porousplate at its lower end and a filler closure at its upper end, a chamberbelow the porous plate and means connecting said chamber to a source ofair under pressure, air operated valve means mounted across the hopperjust above the porous plate and comprising a first cylinder connected atone end to a second cylinder, a piston in said second cylinder andhaving a piston rod coaxial of the first cylinder, said rod having aprojecting end extending into said first cylinder, a coaxial bore insaid first cylinder opposite to said second cylinder, said bore beingengageable by the projecting end of the piston rod to close said valvemeans, a radial bore in the first cylinder for the passage of abrasivematerial from the hopper to the first cylinder, a channel for thepassage of air from the first cylinder to the hopper above the level ofthe abrasive material therein, duct means associated with said secondcylinder to supply air pressure thereto to open and close said valvemeans, and a nozzle connected by channel means to the coaxial bore inthe first cylinder.

positioned adjacent said aperture of the enclosure.

4. An apparatus as claimed in claim 3, in which an air pervious ring ispositioned around the aperture in the wall of the enclosure.

5. An apparatus as claimed in claim 4, in which the enclosure isprovided with a slot in opposite sides thereof, the one open toatmosphere while the other is adapted to be connected to the fan of adust collector unit.

1. An apparatus for correcting misprinted matter on sheet material,comprising means for directing a stream of finely divided particulateabrasive material entrained within a stream of air onto the sheetmaterial, and means for collecting spent material resulting fromimpingement of said abrasive material against said sheet material, anenclosure positionable over said sheet material within which said streamof particulate abrasive material is projected, and means including asource of suction air flow connected to said enclosure; said means fordirecting said abrasive material comprising a storage hopper for saidabrasive material, closure means for said hopper comprising a porousplate at its lower end and a filler closure at its upper end, a chamberbelow the porous plate and means connecting said chamber to a source ofair under pressure, air operated valve means mounted across the hopperjust above the porous plate and comprising a first cylinder connected atone end to a second cylinder, a piston in said second cylinder andhaving a piston rod coaxiAl of the first cylinder, said rod having aprojecting end extending into said first cylinder, a coaxial bore insaid first cylinder opposite to said second cylinder, said bore beingengageable by the projecting end of the piston rod to close said valvemeans, a radial bore in the first cylinder for the passage of abrasivematerial from the hopper to the first cylinder, a channel for thepassage of air from the first cylinder to the hopper above the level ofthe abrasive material therein, duct means associated with said secondcylinder to supply air pressure thereto to open and close said valvemeans, and a nozzle connected by channel means to the coaxial bore inthe first cylinder.
 2. An apparatus as claimed in claim 1, which furtherincludes channel means connected to a source of air pressure passingthrough the storage hopper above the porous plate, and a restrictororifice coaxially inside said channel means, and a nozzle connected tothe outlet end of the channel means.
 3. Apparatus as claimed in claim 2,said enclosure within which said nozzle is mounted having a wall with anaperture therein for the passage of the stream of abrasive materialtherethrough onto sheet material positioned adjacent said aperture ofthe enclosure.
 4. An apparatus as claimed in claim 3, in which an airpervious ring is positioned around the aperture in the wall of theenclosure.